The present invention relates generally to communication systems and in particular to a technique for controlling access to radio channel resources among a number of competing subscriber units.
Multiple access communication systems continue to enjoy wide spread use. Such systems are designed to allow access to limited communication channel resources among a number of different subscriber units. One example of such a system is a cellular telephone system in which a large number of subscribers vie for access to radio channels provided by a limited number of base stations.
The first generation of cellular systems used a multiple access technique known as Frequency Division Multiple Access (FDMA). In such a system, communication resources are considered to be a radio channel of given fixed bandwidth and at a given center carrier frequency. One such a system is the Advanced Mobile Phone Service (AMPS) system which has been widely deployed in the United States. In an AMPS system, access to the communication channels is generally vied for on a demand basis. In other words, channels are provided to users as they switch on their handsets and attempt to make a telephone call. If another user then attempts to place a telephone call and all of the available radio channels are in use, the later user is generally denied access.
Other cellular systems divide the available communication resources in a number of different ways. For example, a noncontention protocol known as Time Division Multiple Access (TDMA) divides the available communication channels into a number of time frames and a number of time slots. Each subscriber unit is then assigned exclusive use of one of the time slots on a given radio channel. This is the basic protocol for radio channel access used by the Global System for Mobile (GSM) communications system widely popularized in Europe.
Another contention-based protocol known as slotted Aloha allows the communication units to actively compete with each other to gain access to the time slots. In this type of system, a subscriber unit desiring to communicate will attempt to transmit in a given time slot which appears to be available. The unit then monitors for a collision. If it detects no other communication subscriber unit attempting also to transmit in that time slot, the system will consider the transmission to be successful. However, if one or more of the subscriber units do collide, all the attempted transmissions are considered to be a failure. In this instance, attempts are made to transmit at a later time, at usually randomly selected time slots.
A further multiple access communication protocol known as Code Division Multiple Access (CDMA) is beginning to be widely deployed in the United States. Using this protocol, individual transmissions are encoded using pseudo-random (PN) and/or orthogonal codes. This permits individual users to transmit at the same time and on the same radio carrier frequency without generating interference with one another. However, the number of active users which may be transmitting at any given time on any given radio channel eventually reaches a noise saturation limit. Therefore, even in CDMA systems, the amount of channel resources must be considered to be limited.
In these cellular communication systems, radio channels are allocated such that they require use of one logical channel exclusively, whether it be defined by the FDMA, TDMA, or CDMA protocols previously described. In general, allocation of the available channels, no matter how they are defined, is contention-based and no preemption is allowed. In other words, the networks do not specifically cater to any particular mobile unit in philosophy when granting access to channels. Rather, requests for access are typically granted in the order in which they are received. While this allocation approach helps maximize the usage of all available resources, it does so at the expense of eliminating the ability to attend to a request from a specific user which may thus otherwise be blocked from network access.